You could just have the client ping the server, if the ping is never received, have the server boot the player.

EDIT: This could also help you mimic the logout-delay stuff MMOs have to prevent people from logging out right before they die. Purposely make the check a countdown of about 20 seconds since the last input was recieved. If 10 seconds pass, and the client doesnt send a ping, log them out.

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